1. Field of the Invention
The present invention relates to novel organometallic preceramic polymers based on boron and nitrogen and to the pyrolysis of such preceramic polymers into ceramic materials at a temperature ranging from 1,000.degree. to 2,000.degree. C.
This invention especially relates to the conversion of such novel preceramic polymers into ceramic materials and shaped articles based on boron nitride, especially boron nitride in fibrous form.
2. Description of the Prior Art
Boron nitride is increasingly in demand in this art in light of its high thermal stability, its resistance to thermal shock, its great chemical inertness and its very good thermal conductivity. Furthermore, its low electrical conductivity makes it an insulator of choice.
Various processes are presently known to the art for the preparation of boron nitride.
One such process entails reacting boron trichloride with ammonia in the gaseous phase. A fine boron nitride powder is thus obtained, which can be sintered to produce solid shaped articles. However, the shaped articles thus produced exhibit a characteristic microporosity which can be highly detrimental for certain applications.
More recently, it was discovered that boron nitride could be prepared by pyrolysis of precursor polymers.
One advantage of this "polymer" process primarily resides in the form of the final product, and, more particularly, enables the production, after pyrolysis thereof, of boron nitride-based fibers.
Thus, in U.S. Pat. No. 4,581,468 a preceramic organoboron polymer is described which is prepared by the interaction of ammonia (ammonolysis) with a trichlorotrialkylsilylborazole (a cyclic compound) and which, as indicated, after drawing and pyrolysis at 970.degree. C., results in the production of boron nitride fibers.
However, the starting cyclic compound described in this patent is very difficult to prepare and is thus expensive. Consequently, it is not suitable for production on an industrial scale.
Published French Patent Application 2,620,455 described a preceramic polymer that is the product of reaction of a mixture of a trihaloborane and of a compound containing at least one boron atom to which two halogen atoms are directly bonded, with a compound containing at least one NH.sub.2 group.
According to published French Application 2,620,443 this preceramic polymer can be pyrolyzed under ammonia to produce a ceramic material essentially based on boron nitride.
The article by K.A. Andrianov, Bulletin of the Academy of Science, USSR, 3, pp. 1,757-1,758 (1962) describes a process entailing introducing a trihaloborane of the formula BX.sub.3 into a hexaalkyldisilazane of the formula (R.sub.3 Si).sub.2 NH to produce a trialkylsilylaminodihaloborane of the formula R.sub.3 SiNHBX.sub.2 ; this reaction product is only a simple organoboron monomer and cannot be used, as such, as a suitable boron nitride precursor.
The reaction of a trihaloborane with a disilazane is also described very generally in EP-A-305,985, and the reaction product obtained is used as a precursor for making ceramized fibers under vacuum or under an inert atmosphere. The product obtained has a melting temperature which can be adjusted depending on the operating conditions of the reaction, but presents disadvantages when it is converted thermally, at temperatures up to 1,000.degree. C., into ceramic materials, in that it comprises a high proportion of silicon and is poorly stable on prolonged exposure to the ambient air.
French Patent Applications 88/13 11, filed Oct. 6, 1988, and 89/05 177, filed Apr. 13, 1989, both assigned to the assignee hereof, describe that the reaction of a trihaloborane with a disilazane must be carried out under special conditions, to ensure that a polymer is produced which can be used as a ceramic precursor.
U.S. Pat. No. 4,707,556 describes the preparation of a ceramic polymer by reacting B-trichloroborazine with a disilazane. The precursor obtained is a solid which is soluble, in particular, in hexane and which presents the disadvantage of having a nonadjustable melting temperature.